Paramecium, a one-celled organism that lives in water. It is common in stagnant ponds, where it feeds on yeasts and bacteria. The paramecium is 1/250 to 3/250 inch (0.1 to 0.3 mm) long. The body is transparent and slipper-shaped, has a groovelike mouth, and is covered with cilia (tiny hairs) used for swimming.

The paramecium contains internal structures that function somewhat like the organs of more complex organisms. It has two nuclei instead of the single nucleus found in most cells. Two contractile vacuoles forcibly expel excess water from the cell. Reproduction is by division of the cell; occasionally, division follows an exchange of nuclear material with another paramecium. Because of its relative simplicity, the paramecium is widely studied in introductory biology courses.

The paramecium was formerly classified as belonging to the animal kingdom, in the phylum Protozoa. It is now usually classified as belonging to the protist kingdom, either in the phylum Protozoa or in a phylum called Ciliata by some authorities and Ciliophora by others. The species usually studied in classrooms is Paramecium caudatum.